Young Ann Lake has recently relocated to England from America alongside her successful, yet overbearing brother Stephen. Their bond is remarkably close, with Stephen showering Ann with affection. On the day Ann is set to enroll her child, Bunny, in a private school, she leaves Bunny in the First Day room under the supervision of the school's female German Cook and rushes off to meet the movers and attend to errands.
When she returns at noon to pick up Bunny, Ann is distressed to discover that no one at the school seems to have any information about her daughter. Seeking answers, she interrogates Elvira, a member of the school's staff, but Elvira is just as puzzled. The situation escalates when Stephen arrives and confronts Elvira for more answers.
Before long, Supt. Newhouse, a composed British police officer, arrives to investigate the perplexing situation. He speaks to everyone involved, including Ada Ford, one of the school's older founders, who resides upstairs and is preoccupied with documenting children's nightmares. Despite tolerating Stephen's rude behavior, Newhouse begins to piece together Ann's profile, leading him to question the validity of her story. The fact that no one has seen Bunny, along with the Cook having vanished, raises doubts about whether Ann ever had a child at all. This insinuation infuriates Stephen, who seems to back it up by mentioning Ann's imaginary friend Bunny from her childhood.
Ann finds herself in a race against time to uncover evidence proving her child's existence while contending with her unsettling landlord, Wilson, an aging actor with a penchant for alcohol and his dog. The police also question him as part of the investigation. When Stephen unexpectedly encounters Newhouse in a pub, he confronts the officer, sharing his accusations.
As the police search for the Cook, Ann's tension escalates. In a moment of desperation, she recalls a claim check for a doll hospital where Bunny's doll has been sent for repairs. Despite the late hour, Ann decides to head there, informing Stephen of her plans. Meanwhile, Newhouse and his team delve into the travel records regarding Ann and Bunny’s journey to England.
Arriving at the doll shop, Ann finds the door unlocked, allowing her access. She retrieves the doll, but Stephen, who has been secretly trailing her, reacts violently. In a fit of rage, he sets the doll on fire, shocking Ann. Stephen then assaults her and leaves her unconscious at the hospital.
However, Ann manages to escape and finally tracks down Stephen to the secluded house they initially lived in upon arriving in England, where he has concealed Bunny. The shocking truth unveils itself: Stephen is the kidnapper! His obsession with Ann has led him to perceive Bunny as a threat to their intense relationship. His mental state deteriorates further, morphing him into a dangerously unhinged figure intent on eliminating Bunny.
In a desperate attempt to rescue Bunny, Ann engages Stephen in nostalgic children's games, managing to evade him with Bunny in tow. Just as Stephen closes in, Newhouse and the police arrive, apprehending the deranged brother. Finally, they acquire the crucial evidence of Bunny's existence through the passenger records from their voyage to England.
